This invention relates generally to paste material dispensing apparatus and particularly to an apparatus for uniformly dispensing high viscosity frit material onto a seal edge of a funnel portion of a cathode ray tube (CRT).
A CRT is composed of a faceplate, or panel, and a funnel portion. A phosphor screen is provided on the inside surface of the faceplate and luminescences when impacted by electrons to produce a visual output. The funnel portion includes a neck to support an electron gun which provides the electrons to scan the phosphor screen. The funnel portion and faceplate panel must be permanently and hermetically joined to permit evacuation of the envelope, as required for operation of the CRT. The panel and funnel are each provided with mating seal edges along which they are joined. A vitrifiable material is placed continually along one of the seal edges. The funnel is placed in a carrier and the panel is placed on the funnel with the mating edges aligned. The carrier supports the funnel and panel as they pass through an oven which is raised to high temperature to vitrify the frit material to permanently and hermetically join the two components.
Typically, the frit material is applied to the sealing edge of the funnel by rotating the funnel at a constant rotating speed. A frit material dispenser, including a hopper and a feed auger, is pivotably arranged above the seal edge. The dispenser includes a follower to retain the dispenser above the seal edge as the funnel rotates beneath the dispenser.
U.S. Pat. No. 4,592,303 describes a paste material dispensing device which operates in the manner described above. The device described in this patent operates quite satisfactorily for many purposes. However, problems sometimes arise because the paste material is dispensed onto the seal edge of the funnel as a bead extending completely around the seal edge. Optimum sealing of the panel to the funnel requires the frit material to be uniformly dispensed so that the beginning and end of the bead meet perfectly with neither a gap between, nor an overlap of the bead ends. The frit paste material comes in two forms. In one form, the frit material is a powder substance which must be mixed into a paste prior to being dispensed onto the seal edge. The uniform on-site mixing of various batches of the frit material is extremely difficult because of environmental changes in temperature and humidity. For this reason, premixed frit material is preferable because the material can be uniformly mixed in a controlled atmosphere. Premixed material also avoids the problems of contamination and inconvenience of on-site mixed materials. However, with existing dispensing devices, the use of the premixed frit material is extremely difficult because the viscosity of the premixed material is substantially higher than that of the on-site mixed material. The substantial increase in the viscosity of the premixed material makes it extremely difficult, if not impossible, to uniformly dispense the premixed material as a uniform bead onto the seal edge. For these reasons there is a need for an apparatus for uniformly dispensing high viscosity premixed frit material onto the seal edge of a CRT funnel such that the paste material forms a complete gapless and overlap free bead around the seal edge. The present invention fulfills these needs.